3GPP Long Term Evolution (LTE) is a standard for mobile phone network technology. LTE is a set of enhancements to the Universal Mobile Telecommunications System (UMTS), and is a technology for realizing high-speed packet-based communication that can reach high data rates on both downlink and uplink channels. As illustrated in FIG. 1, LTE transmissions are sent from base stations 102,110 such as Node Bs (NBs) and evolved Node Bs (eNBs) in a telecommunication network 106, to mobile stations 104, 108 (e.g., user equipment (UEs)). Examples of wireless UE communication devices include mobile telephones, personal digital assistants, electronic readers, portable electronic tablets, personal computers, and laptop computers. The UEs operate within serving cells 112, 114 corresponding to base stations 102, 110, respectively. LTE wireless communication systems may be deployed in a number of configurations, such as, for example, a Multiple-Input, Multiple-Output (MIMO) radio system.
The LTE standard is primarily based on Orthogonal Frequency Division Multiplexing (OFDM) in the downlink, which splits the signal into multiple parallel sub-carriers in frequency. In LTE, available transmission capacity is divided within the frequency domain 206 and time domain 208 into a plurality of resource blocks (RBs). For instance, as illustrated in FIG. 2, a frame 200 comprised of transmission resources (e.g., RBs) 202, 204 may be transmitted in accordance with the LTE standard. Each of resources 202, 204 may consist, for example, of twelve (12) sub-carriers in the frequency domain and 0.5 ms in the time domain.
One aspect of the LTE transmission scheme is that the time-frequency resources can be shared between users. A scheduler controls assignment of the resources among the users (for both downlink and uplink) and also determines the appropriate data rate to be used for each transmission. Due to the use of OFDM, the scheduler can allocate resources for each time and frequency region. For instance, in the example of FIG. 2, a first user may be allocated a first group of resources (shown with hashing) that includes resource 202 while a second user is scheduled for transmission on a second group of resources (shown without hashing) that includes resource 204. In most systems, the condition of the channel for each user is a consideration in determining the most efficient allocation of resources. For instance, a scheduler may be configured to give scheduling priority to UEs with the highest channel quality.
The quality of the signal received by a given user is dependent upon a number of factors, including the channel quality from the serving node, the level of interference from other cells and nodes, and the noise level. In order to optimize the overall capacity of the system, a scheduler will typically try to match the modulation, coding, and other signal/protocol parameters to the signal quality. For instance, when signal quality is low, a scheduler may reduce the coding rate or select a lower-order modulation scheme to increase tolerance to interference and raw bit error rates (e.g., error rates measured before decoding) or to otherwise improve robustness.
According to the LTE standard, UEs may be configured to report Channel Quality Indicators (CQIs) to assist the scheduler. These CQI reports are derived from the downlink received signal quality and are often based on measurements of the downlink reference signals (RSs). Channel Quality Indicators may be referred to as Channel State Information (CSI) in certain systems.
Despite the many advantages of existing LTE schemes and protocols, there exists a significant problem with inter-cell interference and a need to coordinate between cells in order to mitigate the negative effects of interference. The LTE standard is primarily designed to operate under the presumption that the entire spectrum is available in each cell. In other words, that the same time-frequency resources may be used in neighboring cells with limited interference. However, this is not always true in practice, particularly at the cell-edge. Transmissions intended for a first user in a first cell, are often overheard by a second, unintended user in a second cell.
In a heterogeneous network (HetNet), the impact of inter-cell interference can be much higher due to the large difference between the transmit power levels of macro and, for example, pico base stations. A HetNet deployment is illustrated in exemplary network 300, which is shown in FIG. 3. The striped and dotted regions 302, 304 represent the serving area of a pico base station 306. The dotted region 304 represents an area where the received power from the pico base station 306 is higher than that from the macro base station 308. The striped region 302 represents an area where the path loss between UE 310 and the pico base station 306 is smaller than that to the macro base station 308. If the pico base station 306 had the same transmit power as the macro base station 308, the dotted region 304 would be expanded to cover the striped region 302. However, in practice, the transmit power level of a pico base station is typically much lower than that of a macro base station, resulting in a much smaller area of the dotted region 304 shown in FIG. 3.
The striped region 302 is often referred to as the “range-expansion zone” because, from an uplink perspective, the system 300 would still prefer that UE 310 be served by the pico base station 306 within this region. However, from the downlink perspective, terminals at the outer edge of such a range-expansion zone experience very large received power differences between the macro and pico layers. For instance, in the example of FIG. 3, if the transmit power levels are 40 watt and 1 watt, respectively, from macro base station 308 and pico base station 306, the power difference can be as high as 16 dB at UE 310. Thus, if UE 310 is in the range-expansion zone and served by pico base station 306 in the downlink, while at the same time the macro base station 308 is serving UE 312 using the same radio resources, UE 310 would be subject to severe interference from the macro base station 308.
Existing solutions to this type of interference attempt to avoid simultaneous scheduling of transmission to and from UEs at the cell-edge of neighboring cells. In order to support inter-cell interference coordination, information is communicated between nodes using, for instance, the X-2 interface, in accordance with the LTE specification. Each cell can identify the high-power resource blocks in the frequency domain (e.g. in terms of resource blocks) or time domain (e.g. in terms of sub-frames) for its neighboring cells. This allows the neighboring cells to schedule cell-edge users in a manner that avoids these high-power radio resources. Also, reduced power sub-frame approaches may be used. These mechanisms are currently employed to reduce the impact of inter-cell interference in LTE.
Scheduling coordination between cells in existing systems, however, is not coupled with rate control. The main objective of conventional inter-cell interference coordination (ICIC) schemes is to avoid reusing the radio resources that have high transmit power levels in neighboring cells. Such an approach ensures cell-edge user performance at the expense of network spectral efficiency. The reduction in network spectral efficiency can be even worse for frequency domain partitioning schemes (e.g. frequency-reuse factor greater than 1) or time domain partitioning schemes (e.g. the almost-blank sub-frame (ABS) approach considered for heterogeneous networks (HetNet)).
Some mobile terminals, known as “interference mitigation receivers,” have internal interference cancellation capabilities. There are various types of interference mitigation receivers, such as post-decoding successive interference cancellation (SIC) receivers and iterative multi-stage turbo interference cancellation (turbo-IC) receivers.
To fully take advantage of a user terminal's (UE's) interference cancellation capability, a base station should adjust the transmission data rate accordingly. This has been done in the single-user MIMO (SU-MIMO) case, which is illustrated in FIG. 4. The serving base station 402 sends multiple data streams to the same UE 404. The transmission rates of the multiple data streams can be adjusted to account for interference cancellation at the mobile terminal.
For example, the 1st data stream of FIG. 4 can be decoded by UE 404 first, despite the presence of interference from the 2nd data stream. After decoding the 1st data stream, the received signal contributed by the 1st data stream can be cancelled; the cleaned-up received signal is used for detecting the 2nd data stream. The base station 402 adjusts the data rate of the 2nd data stream assuming that the interference from the 1st data stream will not degrade (or have little effect on degrading) the reception quality of the 2nd data stream. Such rate adjustment may in fact take place at the mobile terminal in the process of generating the channel quality indicator (CQI) estimates for the multiple data streams. In this process, the mobile terminal accounts for reduced (or no) interference from the 1st data stream to the 2nd data stream. The estimated CQI's are fed back to the base station, which uses them as the basis for determining SU-MIMO transmission rates. However, as discussed above, interference remains a significant problem in the case of multiple co-channel users served by nodes in different cells or multiple user MIMO (MU-MIMO) systems when multiple users are served in the same cell.
Accordingly, there is a need for improved scheduling and rate control coordination between cells and in MU-MIMO scenarios that accounts for interference cancellation by UEs in order to maximize spectral efficiency.